The present invention relates to the receiving of modulated signals and more particularly to a method of determining useful demodulation parameters of a received signal and in particular frequency, phase and code group timing.
Particularly with coherent modulation and detection schemes, it is necessary for the receiving system to know or to determine with considerable accuracy the frequency of the modulated carrier, its phase, and the time at which symbol intervals begin. In systems employing digital signal processing techniques, the frequency and phase are typically determined as offsets with respect to a local oscillator or clock. The determining of these parameters is conveniently referred to as the "acquiring" of the received signal. In analog signal processing systems, phase-locked loops are commonly employed in getting a lock on and thereby determining the carrier frequency and phase but such systems reach acquisition relatively slowly.
Particularly for burst mode transmissions, it is desirable that the received signal be acquired quickly. Burst mode transmissions are characterized in that the carrier is not continuously present but, rather, is transmitted only when data is to be transmitted. To facilitate the acquiring of a received signal, it is common to transmit a preamble which comprises three successive sections. First, an unmodulated carrier is transmitted for a preselected interval to allow a phase-lock loop at the receiving system to lock on and thereby determine the frequency and phase. Following the first section, a simple alternating pattern of zeros and ones is transmitted to establish symbol timing and modulation characteristics and this section is followed by the transmission of a unique code group which established symbol timing. Once the three sections of the preamble are transmitted, the useful data is transmitted and can be received with reasonable reliability.
As will be understood by those skilled in the art, the inclusion of a lengthy preamble in each transmission necessarily affects the total throughput of the data transmission scheme, particularly in burst mode transmission.
Among the several objects of the present invention may be noted the provision of a novel method for acquiring a received signal; the provision of such a method which does not require the initial transmission of an unmodulated preamble segment; the provision of such a method which does not require the transmission of a section of alternating ones and zeros; the provision of such a method which facilitates the rapid determination of frequency, phase and symbol timing in a received modulated signal; the provision of such a method which facilitates the receiving of burst mode transmissions; the provision of such a method which is highly reliable; the provision of such a method which may be implemented relatively simply and inexpensively. Other objects and features will be in part apparent and in part pointed out hereinafter.